Geomorphology and! Neotectonic Activity of Time Gandak Mega-Fan and Adjoining Areas, Middle Gangetic Plains

Based on remote sensing mapping, field study of morphology and soils, and laboratoty analysis of soils, the Gandak mega-fan and adjoining areas are divided into ten major geomorphic units - Gandak Floodplain, Young Gandak Plain, Older Gandak Plain, Oldest Gandak Plain, Old Rapti Plain, Old Ghaghra Plain, Ganga-Ghaghra Interfluve Plain and floodplains of the Ganga, Ghaghra and Rapti. The present study indicates that the Gandak river has shifted over its mega-fan from west to east from the Oldest Gandak Plain to Gandak Floodplain over a distance of about 105 km. One way shifting is due to slow eastward neotectonic tilting of the mega-fan. As the Gandak shifts across the mega-fan, it leaves behind numerous ponds, channels, lakes etc., which, with time, get integrated into fewer channels and drainage improves. Slow tilting has resulted in deposition of a flick ( > 1.7 m) vertical accretion silt layer over most of the mega-fan, even on the proximal portion deposited by the braided Gandak Channel. It has also been responsible for higher degree of soH development from east to west on the Gandak mega-fan. The whole region of the Gandak mega-fan is subsiding, but in the area of the Young Gandak Plain the sedimentation rate is slow and is not keeping pace with subsidence. This has resulted in impeded drainage, waterlogging and ponding on a large scale.